body, reducing the DANC. GIS tools could be used to evaluate representative watersheds throughout the country to determine the effective length of Bin 3 and 4 networks. Although this evaluation could not be conducted in the given time frame, it may be considered in future modeling efforts.

####### 3.1.4.2.1.4. Spreading Out Applications

Currently, PRZM5/VVWM assumes applications occur on fixed dates, specified by the user as absolute 12th) dates (e.g., February or dates relative to crop emergence. Usage data obtained from California’s Pesticide Use Reporting system and other pesticide use surveys show that for large watersheds not all farmers apply a pesticide on the same date, every day for 30 years. Applications in a watershed would be spread out over a period of time, based on pest pressure, relative planting date, precipitation events, and availability of the applicator. To account for this process, spreading out an application event over a fixed number of days has been explored. Retreatment intervals, may also affect the days over which applications can be made. Some preliminary modeling has been conducted with diazinon to evaluate the impact of spreading out the applications over time. A single application was spread uniformly, using the retreatment interval as the fixed window of application. Instantaneous peak EECs are only slightly reduced. Further analysis indicates that the fixed window is not long enough to allow for significant degradation to occur in the soil between applications. Thus, the loading to the water body is the same as if the application occurred on a single day. Additional evaluation of this approach is needed to identify the appropriate application window, annual maximum application rate, and retreatment intervals for diazinon. The SAM has shown improved correspondence in the timing and magnitude of modeled and measured concentrations, by varying application dates each year according to crop growth stages, and spreading out applications over multiple days. Crop growth stages (e.g., planting, harvest) vary spatially and temporally, but only limited empirical information is available to define the growth stages for all crops nationally. Preliminary evaluations of this approach for atrazine applied to corn were supported by the Scientific Advisory Panel, and the Panel additionally recommended the use of other empirical data (e.g. crop.18 crop insurance) when USDA Crop Progress reports are unavailable for a specific

###### 3.1.4.2.2. Modifications Explored and Incorporated into Modeling

- The following changes were explored and incorporated into the case study at the end of this section.

####### 3.1.4.2.2.1. Curve Number Adjustment

Curve numbers are selected for use in PRZM5/VVWM based on soil hydrologic group, land use, agricultural practices, and the hydrologic condition of the soil. Traditionally, the combination of soil, land cover, cropping, and management practices (e.g. conventional tillage, no-till) for particularly vulnerable sites results in high runoff curve numbers. While assuming a high runoff curve number is acceptable when modeling a representative field, the use of the same curve number for an entire watershed is a simplification, given the variety of land uses, hydrological conditions, and hydrologic soil groups within a watershed. A more refined approach would be to model fields and untreated areas within a watershed separately, each with an appropriate curve number. Although PRZM5/VVWM is not